Data rich imaging approaches assessing early fatigue crack growth mechanisms in Ni base superalloys

镍基高温合金由于其优异的高温强度、抗氧化性和损伤容限而用于涡轮机盘应用中。在使用中，它们在相对高的应力下经历极端的温度和循环载荷条件，因此它们的疲劳寿命大部分由裂纹萌生和生长的早期阶段控制。镍基高温合金中裂纹尺寸的变化对裂纹萌生和扩展的影响通常与滑移特性的变化有关。损伤的累积本质上受滑移不均匀性程度的影响，这会影响启动和增长过程。因此，温度、环境和与尺寸相关的滑移特性的综合影响通过对起始过程和裂纹扩展机制的不同影响来控制总体疲劳寿命。大多数在役涡轮机盘合金具有双峰或三峰“尺寸分布，这里，”第一“尺寸（或存在）与固溶热处理的持续时间和温度（例如，超固溶线或亚固溶线）有关，而”第二“和”第三“尺寸与时效温度和冷却速率有关。这些不同大小的种群对滑动特性的影响和相对平衡是一个持续争论的问题。在这项工作中，非商业热处理已被用于RR 1000合金，以产生两个不同的单峰的尺寸分布。这包括超固溶线热处理，随后是单次时效和缓慢冷却，以分别产生细尺寸和粗尺寸。用三点弯曲法对细、粗材料抛光光弯曲棒材在屈服应力为90%和110%、载荷比为0.1、频率为20Hz的正弦波形下进行了室温短裂纹疲劳试验。本文用重复法进行了不间断和间断的裂纹扩展试验，并与南安普顿大学以前的工作进行了比较。普通弯曲杆测试表明，具有细变体的RR 1000具有比具有粗变体的RR 1000稍长的疲劳寿命，这与较慢的I阶段裂纹扩展有关，这与室温下更多的平面滑移有关。室温下裂纹主要在孔隙或滑移带处萌生，细晶试样的疲劳裂纹扩展速率略高于粗晶试样。3D评估已提供的X射线CT提取火柴棒在大学的南安普顿，并通过检查裂纹萌生和微拉伸样品的增长。通过扫描获得的二维切片，研究了RR1000粗钢内部的显微组织，包括气孔分布和裂纹行为。通过结合3D X射线计算机断层扫描和传统的2D观察，这提供了对该模型涡轮机盘合金在一系列服务相关应用下的疲劳裂纹萌生和扩展行为的机制的更深入的理解。

Nickel base superalloys are used in turbine disc applications because of their excellent high temperature strength, oxidation resistance and damage tolerance. In service they experience extremes of temperature and cyclic loading conditions at relatively high stresses and so much of their fatigue life is controlled by the early stages of crack initiation and growth. The effect of varying' size on crack initiation and growth in nickel based superalloys is often linked to variation in slip character. The accumulation of damage is intrinsically affected by the degree of slip heterogeneity and this can affect both initiation and growth processes. Thus the combined effects of temperature, environment and slip character linked to' size control overall fatigue life through varying effects on initiation processes and crack growth mechanisms. Most in-service turbine disc alloys have a bi-modal or tri-modal' size distribution, here primary ' size (or presence) is linked to the duration and temperature of the solution heat treatment (e.g. super or sub-solvus) whilst secondary' and tertiary ' size are linked to the ageing temperature and the cooling rates. The effect and relative balance of these different ' size populations on slip character is a matter of continuing debate. In this work a non-commercial heat treatment has been used on RR1000 alloy to produce two different unimodal ' size distributions . This comprised a super-solvus heat treatment followed by a single age and slow cool to produce fine and coarse' sizes, respectively. Short fatigue crack tests have been conducted on polished plain bend bar of fine and coarse' materials by three point bending method under 90% and 110% of yield stress, load ratio of 0.1 and a sine waveform of 20Hz frequency at room temperature. Uninterrupted and interrupted tests with replication ethod to investigate crack evolution were carried out and compared with the previous work done by the University of Southampton. Plain bend bar tests showed that the RR1000 with fine variant had a slightly longer fatigue life than RR1000 with coarse ', linked to slower Stage I crack propagation linked to more planar slip at room temperature. Cracks mostly initiated at pores or slip bands at room temperature and fatigue crack growth rate of fine ' is slightly higher that of coarse'. 3D evaluation has been provided by X-ray CT of extracted match-sticks at University of Southampton and by inspecting crack initiation and growth in micro-tensile samples. Internal microstructure analysis of RR1000 with coarse' including porosity distribution and crack behavior has been investigated from a series of 2D slices obtained by scanning. This provided a deeper understanding of the mechanisms of fatigue crack nitiation and propagation behaviour in this model turbine disc alloy under a range of service relevant applications by the combination of 3D X-ray computed tomography and traditional 2D observations.